The present invention relates generally to high energy propagation devices, and more particularly to a lens having an adjustable back focus.
Lenses are typically utilized in systems which require focusing light at a predetermined location referred to as a primary focal point. For a typical lens, the light incident on a first surface of the lens propagates through the lens and exits the lens through a second surface of the lens. The surfaces of the lens are configured to focus the light at the primary focal point. A portion of the light which is incident on the first and second surfaces of the lens is reflected back from the lens. Typically, the first surface of the lens is convex in shape to the incident light resulting in the light reflected off the first surface being dispersed. On the other hand, the second surface of the lens is typically concave in shape to the incident light resulting in the light reflected off the second surface focusing at a back focal point.
Typically, an anti-reflective coating is applied to the surfaces of the lens to minimize the amount of light reflected from the first and second surfaces. The anti-reflective coating typically reduces the amount of back reflected light to as small as one tenth of one percent of the incident light. This small amount of reflected light is inconsequential to the operation of the typically system and is ignored for most lens applications. However for high energy applications, such as high power lasers, even this small amount of light, when brought to a focus can damage and eventually destroy components located at the back focal point.
Due to the optics of the laser, the designer is limited as to where he can locate the lens. Since the lens can not be relocated to move the back focus, laser designers are constrained to position components to avoid locating them coincidence with the back focal point. This constraint can greatly restrict the design of the laser and result in a larger, less compact laser. What is needed therefore is a lens having an adjustable back focus such that the components of the laser can be placed at the most desirable location and the back focus of the lens can be adjusted to move the back focus location away from sensitive components while maintaining the location of the primary focal point of the lens.